1. Technical Field
The present disclosure relates to an ultrasonic testing device and an ultrasonic testing method by which a workpiece (or an object) is tested using ultrasonic waves, and particularly to art effective in blowhole detection of a workpiece that normally have an air gap at a weld toe.
2. Related Art
Casings of differentials (i.e., differential casings) provided as final drive units used for vehicles, such as automobiles, have a structure in which, for example, a ring gear of a hypoid gear set is joined to the outer circumference of the differential casing.
The ring gear is joined to the differential casing by welding, and a welding technique, such as laser welding, is widely used for this purpose. Laser welding is a joining technique to irradiate a laser beam to a joining part between the differential casing and the ring gear, thus the joining part is molten and then solidified.
Inspections (or tests) of the weld quality is then conducted, for example, by using an ultrasonic testing device for each piece of the differential casing onto which the ring gear is welded. The ultrasonic testing device is as device that detects a blowhole, i.e., a minute defect that is comprised of a spherical or almost spherical hollow part caused in deposited metal.
Ultrasonic testing is a technique to test (or evaluate) a defect based on an ultrasonic wave reflected back from the defect. The differential casing onto which the ring gear is welded is first immersed into liquid filled in a liquid tank. An ultrasonic wave is then transmitted from an ultrasonic probe that is similarly immersed in the liquid of the liquid tank, and the ultrasonic wave reflected on the defect is received by the probe for the evaluation. The differential casing and the ultrasonic probe are immersed into the liquid in order to prevent the ultrasonic wave from being reflected on air layer.
For example, Japanese Unexamined Patent Application Publication (JP-A) No. 2001-201485 addresses an improvement in accuracy of the evaluation for the type of ultrasonic testing technique with the workpiece immersed. Specifically, when the workpiece is immersed into liquid (water) of a liquid tank, a large temperature difference between the workpiece and water where the workpiece is immersed is prevented to be caused, and thereby improving the accuracy.